The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for preventing a belt from slipping on pulleys of the belt drive transmission.
A known control system for a continuously variable belt-drive transmission connected to an electromagnetic clutch disclosed in Japanese Patent Application Laid-Open No. 54-157930 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of a belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine.
The line pressure control valve is operatively connected with a sensor shoe through a lever and a spring. The sensor shoe is shifted in accordance with the transmission ratio, thereby changing the elastic force of the spring. Thus, the line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed, that is, pitot pressure. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At starting of the vehicle, the transmission ratio is set at a maximum value. When the engine speed exceeds a predetermined value, the electromagnetic clutch is engaged, so that the vehicle is started. When the vehicle speed and engine speed exceed set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced, even if the engine speed is kept constant. Thus, the vehicle speed increases with the reduction of the transmission ratio.
However, the line pressure is controlled in accordance with the transmission ratio and the engine speed, and not dependent on changes of other driving conditions.
For example, when an accelerator pedal of the vehicle is rapidly depressed during driving on a road having a low friction coefficient, wheels of the vehicle may spin. If wheels spin, although the engine speed Ne and hence the wheel speed Vw increase, the vehicle body speed Vm hardly increases as shown in FIG. 6a. The increase of the engine speed Ne causes the pitot pressure to rise so that the transmission ratio i reduces (upshift), thereby decreasing line pressure P.sub.L (FIG. 6b). When the driver, noticing the spinning of the wheels, releases the accelerator pedal (FIG. 6c), the engine speed Ne and the wheel speed Vw rapidly decrease. As a result, the wheels come into gripping of the road. The transmission is upshifted to a minimum small transmission ratio because of the reduction of the engine speed Ne. Accordingly, the line pressure P.sub.L becomes a minimum, so that the belt gripping force of the pulleys reduces. On the other hand, the speed of the driven pulley rapidly reduces because of the road gripping of the wheels. Such a sudden change in the rotational speed of the pulleys causes the belt to slip on the pulleys.